1. Field of the Invention
This invention relates to information recording and more particularly, to high density information records of a variable electrostatic capacitance form, such as video or audio discs, on which signal information is recorded as geometric variations.
2. Description of the Prior Art
In certain information playback systems of the electrostatic capacitance type, signal information is recorded as geometric variations or pits in a spiral plane or groove or in concentric planes or grooves by compression molding a conductive resin composition between stampers having modulated ridges on the surfaces thereof, thereby obtaining a disc with an impression of the stampers therein as signal information.
The individual geometric variations or pits of the disc are very small and are arranged in a very high density. When the geometric variations are traced with a pickup stylus such as of diamond having an electrode, capacitive variations are established between the stylus and the disc due to the geometric variations therein, thereby playing back pictures and sounds. Typical of the playback system is a video disc of the electrostatic capacitance type.
Such a disc should have various good characteristics or properties such as signal-to-noise ratio showing a reproduction level of signal, durability against repeated cycles of reproduction, mechanical strength, heat stability, humidity proof and the like, so that the performance of the disc is maintained over a long term. To satisfy the above requirements, various conductive resin compositions for discs and molding techniques have been heretofore proposed. At present, it is generally accepted that resin compositions comprising vinyl chloride resins, carbon black with a large surface area and, if necessary, stabilizers, lubricants and the like are most preferable from the practical standpoint. Molding techniques of making discs from these resin compositions are advancing but a serious problem remains in making of the disc in that fine defects such as sharp projections, gentle blisters and the like (hereinafter referred to simply as blister) are formed on the surfaces of the disc.
As mentioned before, signal information is impressed in the disc as fine geometric variations of very high density. For instance, 50 billions of signal pits each with a depth of 0.3 of 0.4 .mu.m are impressed with a track pitch of 1.35 .mu.m. The pits are concentrically or spirally arranged in a disc surface with the innermost track turn having a diameter of 9 cm and the outermost track turn having a diameter of 25 cm. Adjacent pits have intervals of about 0.6 .mu.m at the innermost track turn and of about 1.7 .mu.m at the outermost track turn. Thus, geometric variations are very fine. When blisters are formed on the disc surface, formation of signal pits are adversely affected with the smoothness of the disc surface being impeded. This presents a serious problem in the fabrication of video or digital audio discs. More particularly, during playback of recorded signal information, a diamond stylus having an electrode is slidably contacted with the concentrically or spirally arranged signal pits while controlling by tracking signals as proposed by Victor Company of Japan, Limited or is slidably contacted with a spiral groove in which signal information is impressed at the bottom thereof as proposed by RCA Co., Ltd. U.S.A. The relative velocity of the stylus and the disc reaches as high as 10 m/second. If the disc has blisters thereon, the stylus will jump at the portion of blisters from the disc surface. Signal information cannot be accurately picked up, leading to a defect of a reproduced picture or image (hereinafter referred to dropout). Alternatively, the jumped stylus may scrape off part of the disc body upon contact with the disc surface, so that signals contained in the scraped-off portion are erased, thereby increasing dropouts or generating dust which is unfavorable for the disc. In an extreme case, when the stylus drops on the disc after the jumping, it may move toward the direction right-angled with the tangential direction of the portion where dropped thereby dropping out signals jumped or skipped over.
The dropouts caused by a failure in formation of only a small amount of signals or by very small-size blisters with several .mu.m may be recovered by electrical correction of a reproducing apparatus. However, dropouts resulting from blisters of a size larger than 10 .mu.m are very difficult to correct electrically. In practical applications, it is necessary to solve the problem of blisters.
A number of factors will take part in formation of blisters. Resin compositions used as starting materials for disc include vinyl chloride-base resins, carbon black, heat stabilizers for preventing thermal decomposition of the resin, flow regulators for improving moldability or processability, molding aids, and/or lubricants permitting smooth contact between disc and pickup stylus. These compositions are uniformly mixed in a high speed mixer, kneaded in melted state, pelletized and compression molded by extrusion or injection compression molding to obtain discs.
Blisters of the thus obtained disc may be derived, for example, from the materials of the resin composition, materials incorporated during the fabrication process, or materials produced by thermal decomposition. Blisters derived from inorganic matters may be reduced by removing foreign solid matters from the fabrication system or treating inorganic materials, e.g. carbon black, by a suitable manner. By this, it was possible to reduce blisters to an extent. However, not larger than about 100 blisters derived from organic matters per disc were still found. Organic matters constituting the blisters may be derived from various sources such as vinyl chloride resins, and various organic additives such as dispersants, lubricants and the like. Of these, foreign matters contained in vinyl chloride resins were found to be most serious.
In ordinary applications, the permissible number of foreign matters in vinyl chloride resin is 100 to 1000 per 100 g of the resin. For applications to video or digital audio discs, such number should be as small as from 0.01 to 1 per 100 g of the resin.